A HetNet (Heterogeneous Network) is a technology of allocating a plurality of small cells in an overlay configuration in a macrocell area for improving a decrease in throughput of user data concomitantly with an increase in the number of users in a mobile communication system for only a conventional macrocell.
The HetNet mobile communication system includes a macrocell base station to provide a large zone area (macrocell area), and one or more small cell base stations to provide one or more small zone areas (small cell areas), which are geographically overlaid on the macrocell area.
In the HetNet mobile communication system, a user terminal generally is in the macrocell area and receives communication services from the macrocell base station forming the macrocell. In contrast, the user terminal, when located in the vicinity of the small cell area, performs handover (off-load) to the small cell area. The user terminal receives the communication services from the small cell base station forming the small cell when residing in the small cell area, and is thereby enabled to perform data communications with a high throughput.
For further information, see Japanese Laid-Open Patent Publication No. 2012-39168, Japanese Laid-Open Patent Publication No. 2012-5080, Japanese Laid-Open Patent Publication No. 2011-223113, Japanese Laid-Open Patent Publication No. 2012-104950, and Japanese Laid-Open Patent Publication No. 2011-238981.
In the conventional HetNet mobile communication system, the user terminal being in the macrocell area but not being in the small cell area executes a cell search based on “peripheral cell information” (also called “adjacent cell information) contained in notifying information received from the macro cell. In the macrocell area, information of all of the small cell areas existing within one macrocell area is notified as the “peripheral cell information” to the macrocell area throughout.
On the other hand, there is ununiformity in allocation of the small cells within the macrocell area, i.e., there are a region with the small cells being densely located in the macrocell area and a region with any small cell not existing as the case may be. In this case, when the “peripheral cell information” is notified by the foregoing notifying method, it follows that the user terminal located in the region with any small cell not existing continues executing the cell search in spite of the small cell not actually existing in the vicinity of the present location. This continuation of executing the cell search in an environment with a low success rate brings about a waste of electric power.